1. Field of the Invention
The invention relates to a circuit arrangement for igniting thin rods comprising electrically conductive material, i.e., thin silicon rods. The invention also relates to the use of a three-phase AC system (three-phase system) for feeding electrical energy to the thin rods comprising the electrically conductive material, i.e., the thin silicon rods.
2. Description of the Related Art
EP 2 234 259 A1 discloses a circuit arrangement for igniting four thin silicon rods in a reactor for producing silicon rods from the thin silicon rods according to a Siemens process.
In accordance with the known Siemens process, metallurgical silicon is firstly converted to liquid trichlorosilane using gaseous hydrogen chloride at approximately 600K.
The trichlorosilane is then purified in a plurality of distillation steps, where vapor deposition then ensues.
During this vapor deposition, the distillate is caused to boil and is conducted over hot thin silicon rods in deposition reactors, designated just as reactors for short hereinafter. Silicon crystals then grow on these hot or heated thin silicon rods, where the trichlorosilane is decomposed with the aid of hydrogen to form silicon, hydrogen chloride and silicon tetrachloride.
The growth can be begun anywhere on the hot thin silicon rods. As a result, the entire (thin) rods are then not monocrystalline, but rather consist of a plurality of crystals (polycrystalline silicon).
After approximately one week, the (polycrystalline) thin silicon rods have grown to the desired size, i.e., to the silicon rods, and are withdrawn from the reactor or exchanged.
The heating of the thin silicon rods in the reactor is effected electrically, where voltage is supplied or fed to the thin silicon rods.
During a start phase, i.e., ignition of the thin silicon rods and run-up of the reactor, the thin silicon rods are supplied with a medium voltage, which is greater than 2500 V. Ignition of the thin silicon rods can then be achieved with such high voltages.
Once all the thin silicon rods have been ignited, i.e., the start phase has ended, an operating phase ensues, in which the thin silicon rods are supplied with a voltage that is below the voltage in the start phase. In the operating phase, the thin silicon rods are then grown to form the (polycrystalline) silicon rods.
The high voltages required/used for igniting the thin silicon rods introduced in the reactor would require complex insulations in the reactor for the shielding thereof. Accordingly, recourse is had here to specific circuit arrangements in the context of the voltage supply of the thin silicon rods, with thin silicon rods being correspondingly arranged and interconnected in the reactor.
The specific circuit arrangements are based on three-phase or AC/AC voltage systems that are interconnected such that the voltages applied to the thin silicon rods or made available to the thin silicon rods by the circuit arrangement cancel one another out.
EP 2 234 259 A describes one such specific circuit arrangement, via which four thin silicon rods can be ignited. In this, the circuit arrangement therein or the three-phase/AC voltage system therein is interconnected such that the four thin silicon rods are fed from respective single-phase systems with voltages offset by 180° with respect to one another.
What proves to be disadvantageous about these circuit arrangements is that they are only suitable for igniting four (or a multiple of four) or only an even number of thin silicon rods correspondingly arranged relative to one another. In other words, these circuit arrangements are only insufficiently suitable for also igniting a number of other than four (or a multiple thereof) thin silicon rods correspondingly arranged in relation to one another or an odd number of thin silicon rods correspondingly arranged in relation to one another.
A three phase transformer is disclosed in the German version of Wikipedia.
Three-phase transformers, as a specific form of a plurality of combined single-phase transformers, which single-phase transformers set out in their general embodiment to decrease/increase voltage, are used for electrical energy transfer by means of stepped-up three-phase AC current.
In this case, such a three-phase transformer combines the three individual single-phase transformers required for (step-up) transformation in a three-phase system to form a single transformer.
Three-phase transformers are constructed similarly to the single-phase transformers. In contrast to the single-phase transformers, however, here, i.e., in the case of the three-phase transformers, a primary side and secondary side each consist of three separate windings, usually designated by U, V, W for the high-voltage side and u, v, w for the low-voltage side, which are accommodated on a soft-magnetic iron core having three or five legs. Here, the high—voltage windings and low-voltage windings are usually fitted on a respective limb in a manner isolated by corresponding insulation material.
The way in which the (in total) six connections per side (i.e., the primary and secondary sides) of a three-phase transformer are interconnected with one another is determined by a vector group.
Customary interconnections in the vector groups comprise a star and/or delta connection (winding), which in principle can be combined arbitrarily on both sides.